Can we create a Reusable task in Workflow Designer?
The answer is yes.
When you create a task in workflow designer its non-reusable by default. But you can always promote this to a reusable task. This action cannot be reversed.
Creating a Task in the Workflow or Worklet Designer
You can create and add tasks in the Workflow Designer or Worklet Designer as you develop the workflow or worklet. You can create any type of task in the Workflow Designer or Worklet Designer. Tasks you create in the Workflow Designer or Worklet Designer are non-reusable. Edit the General tab of the task properties to promote a non-reusable task to a reusable task.